1. Field of the Invention
The present invention relates to a spiral multilayer-type filter element constructed by winding a belt-like conductor that has been placed on a belt-like insulator.
2. Description of the Related Art
As examples of this type of filter element, filter elements disclosed as a comparative example and an embodiment in Japanese Patent No. 2,826,320 are known. These filter elements are formed by winding a plurality of belt-like conductors (belt-like conductors for conducting and connecting to ground), via a dielectric sheet with high dielectric constant, around a magnetic or non-magnetic spool or an air core, and function as three-terminal noise filters (normal mode noise filters). In these filter elements, an inductance is formed between a pair of leads connected to a winding start part and a winding end part of the belt-like conductor for conducting, and a capacitance that is distributed constant is also formed between the belt-like conductor for conducting and the belt-like conductor for grounding along the entire length of the belt-like conductor for conducting. With these filter elements, it is therefore possible to realize superior attenuation characteristics across a wide bandwidth.
Out of these filter elements, as shown in FIG. 1(A) of the reference, the filter element (hereinafter also simplified to “filter”) disclosed as a comparative example includes a tube-like wound part and three leads that are provided on one end surface of the wound part. In this case, the wound part is formed by winding a multilayer structure (see FIG. 2 of the reference), in which a belt-like conductor and a grounding belt-like conductor are placed on top of one another with two dielectric layers in between, a plurality of times around a rotating winding shaft. On the other hand, a first inner lead for inputting and outputting is connected to a periphery of an inner end (the left end in FIG. 2) of the belt-like conductor. A first outer lead for inputting and outputting is connected to an outer periphery of the belt-like conductor. In addition, a grounding outer lead is connected to a periphery of an outer end of the grounding belt-like conductor so as to be displaced by a ½ turn about the winding axis from the first outer lead.
With this construction, in the filter, the gap between both leads can be sufficiently narrowed in a state where the conductors are unwound, so that superior attenuation characteristics can be realized. Also, in this filter, since it is possible to position both leads a predetermined distance apart on a substantially straight line that passes through the center of the winding axis, superior mass-producibility and substantially uniform and favorable attenuation characteristics can be realized. Also, as shown in FIG. 1(B) of the same reference, by heating the tube-like wound part in a state where the winding shaft has been removed and simultaneously pressing the wound part from a direction perpendicular to wires connected to the two leads, it is possible to achieved the same effect in a construction where the air core part and wound part have deformed into a flattened shape.
On the other hand, as shown by FIG. 4 of the reference, the filter element disclosed as an embodiment includes a tube-like wound part and three leads provided on one end surface of the cylindrical wound part, with the grounding outer lead being disposed at a position displaced by one half turn about the winding axis with respect to the first outer lead (that is, the first outer lead, the grounding lead and the center of the winding axis being disposed on a straight line) in the same way as the filter element of the comparative example described above. On the other hand, the filter element disclosed as the embodiment differs to the filter element of the comparative example described above in that the first inner lead is disposed so as to be displaced by one quarter turn about the winding center axis with respect to the first outer lead (that is, the first inner lead is disposed on a straight line that passes through the center of the winding axis and is perpendicular to the above straight line).
With this construction, in the filter element disclosed as the embodiment, it is possible to sufficiently narrow the distance between the two leads in a state where the conductors are unwound, so that superior attenuation characteristics can be realized. In addition, when the wound part of the filter element is deformed into a flattened shape, it is possible to position the first inner lead at substantially the same position as the center of the winding axis, and as a result it is possible to dispose the grounding lead and the first outer lead with left-right symmetry about the winding axis, that is, about the first inner lead. Since sufficient physical gaps are obtained between the first outer lead and the first inner lead and between the first inner lead and the grounding lead, it is possible to effectively avoid short circuits inadvertently caused during operations such as wiring and soldering. The filter element can therefore easily be attached to a printed circuit board.
However, an air-core device of this type has little effect on low frequency noise, such as noise produced by a switching power supply. Also, there have been demands for a filter element that has more superior attenuation characteristics. In order to satisfy such demand, the inventors of the present embodiment conducted original research into this kind of filter element, and by using a special arrangement for the respective leads, discovered that a filter element with even more superior attenuation characteristics can be realized.
The main application of this kind of filter element, in other words, a spiral multilayer filter element where a signal transmitting line and a grounding line that are both formed in belt shapes are wound in a spiral with a tape-like insulator in between, is the eradication of power supply noise caused by a switching regulator or the like, but aside from this, operation as a transformer was also verified.
In a spiral multilayer filter element, the signal transmitting line is held within the grounding line with the insulator in between, so that the signal transmitting line and the grounding line are magnetically coupled to one another and have a large distributed inductance. When the grounding line is examined, however, at the opposite end to the grounded point, an inductive impedance is present on the path to the grounded point due to distributed impedance of grounding line itself, and as a result, the ground potential (0V) is not necessarily achieved along the entire length of the grounding line.
In this way, since the potentials of the parts of the grounding line present on both sides of the signal transmission line are asymmetrical, the operation of the spiral multilayer filter element is extremely complex, but the arrangement of the lead terminals can be given as one factor influencing the performance and characteristics of the filter element.
That is, when a signal transmitting line and grounding line are wound with an insulator in between, a signal input lead terminal and a signal output lead terminal are connected at both ends of the signal transmitting line and one end of the grounding line is connected to the grounding lead terminal, with the positions of the three lead terminals after winding having a large influence on the performance and characteristics of the filter.
To obtain superior attenuation characteristics across a wide bandwidth, the three lead terminals, that is, the signal input lead terminal, the signal output lead terminal, and the grounding terminal, need to be aligned on a straight line with predetermined gaps in between. However, it is extremely difficult to wind the signal transmitting line and the grounding line so that the three lead terminals are aligned on the same straight line.
For this reason, in the three-terminal noise filter disclosed in Japanese Patent No. 2,826,320, when the belt-like conductor (signal transmitting line), to which the first inner lead and the first outer lead for inputting and outputting are connected at peripheries of both ends, and the grounding belt-like conductor (grounding line), to which a grounding outer lead is connected at an outer end periphery, are wound with an insulator sheet in between, the first inner lead and the first outer lead are displaced by one quarter turn, and the first outer lead and the grounding outer lead are brought close to one another in the circuit layout in a state where the leads are substantially one half-turn apart. In addition, by pressing and flattening the wound multilayer structure from a direction perpendicular to a straight line that joins the respective outer leads, the three leads can be placed on a straight line with substantially equal gaps in between.
However, since the positions of the three leads are fixed in the filter disclosed in the above reference, in a circuit board on which the filter is mounted, there are limitations on the design in that through-holes, peripheral components, and the like have to be designed in accordance with the lead positions of the filter.
This kind of filter acts as a normal mode noise filter when there is one signal transmitting line and as a common mode noise filter when there are two (or more) signal transmitting lines, but in either case, if the filter has an air core like the filters disclosed in the reference, it is not possible to obtain a high attenuation effect for low frequency noise in particular.
In a case where the filter element is used as a common mode noise filter for a switching frequency of around several tens to several hundred kHz, such as with a switching power supply, to achieve a sufficient noise suppressing effect, it is necessary to reinforce the magnetic coupling between the respective signal transmitting lines using a magnetic body.
To do so, as the above magnetic body, it is possible to use a pot-type magnetic core or the like that is made of ferrite and includes a center core disposed at a center of a multilayer coil, but the leading of lead terminals from the magnetic core thereof is problematic.
That is, although lead holes and the like are formed in the magnetic core in accordance with the positions of the three lead terminals connected to the multilayer coil, in the actual assembly process, slight positional displacements occur. Also, when ferrite is used as the magnetic body, the ferrite is brittle and so difficult to machine, which means that depending on the positions at which holes and the like are formed, there are cases where the core breaks during the machining process.
Accordingly, in a spiral multilayer filter element, when additionally using a magnetic core that improves the attenuation characteristics for low-frequency noise, there is the problem of how to lead the three lead terminals that are connected to the coil from the predetermined positions to the outside of the magnetic core in accordance with the form of the magnetic core.
In addition, the filter disclosed in the above reference functions as a three-terminal noise filter (a normal mode noise filter) that has three leads, but there is demand for the realization of a spiral multilayer filter element that is a common mode filter element. For this reason, as shown in FIG. 33, the inventors of the present invention fabricated a common mode filter element 1001 by using two conducting belt-like conductors (signal transmitting belt-like conductors) 9a, 9b, disposing the respective signal transmitting belt-like conductors 9a, 9b in parallel on a belt-like conductor (grounding belt-like conductor) 8 for grounding with a belt-like insulator 7b in between, arranging the belt-like conductors 9a, 9b in parallel in the width direction of the grounding belt-like conductor 8, and winding these component parts together. However, in the filter element 1001 of this construction, when the noise attenuation characteristics (as one example the power supply terminal interference voltage characteristics) were measured, it was difficult to achieve satisfactory characteristics. For this reason, there is the problem of how to realize a spiral multilayer common mode filter element with favorable noise attenuation characteristics.